Currently, the development of both ink and media allows an aqueous inkjet recording system to attain high image quality in a level of photo image quality, and many users use the aqueous inkjet recording system for this reason. However, in the commercial print field, the aqueous inkjet recording system has not been widely used, but used in a limited field, such as printing of address of direct mails and ledger sheet, proof output for confirming conditions of offset print, and the like. This is because as paper (medium) special paper which is designed to increase absorption of the inkjet ink is necessary to output high quality image print in the aqueous inkjet system.
In a commercial printing field where a printing cost, speed, and image quality are severely demanded, coated papers (coated paper for commercial print) are generally used for offset printing or gravure printing, in any printing methods. These media are very inexpensive, but not designed for the aqueous inkjet recording method. An inkjet recording method using the aqueous ink, in which image quality comparable to commercial printing can be output on those media at adequately high speed, has not been achieved yet. When the aqueous inkjet printing is performed on the common coated paper for commercial printing, many problems occur, for example, image bleeding and dry failure, due to the poor ink absorption ability and wettability of the coated paper. Therefore, currently the aqueous inkjet recording method cannot be practically used in the commercial printing field.
Recently, matte and dull media are preferably used in high grade printing in terms of viewability of characters or images and their texture. However, a print is demanded to be finished so as to have a matte background and a glossy image in order to exhibit high grade image quality for catalogs of merchandise. In offset printing, image gloss is usually controlled in such a manner that the image gloss can be exhibited in a background without gloss by effectively designing an ink, on the other hand, the image gloss can be lost by mixing medium with an ink. In aqueous inkjet recording, the image gloss is exhibited by use of so-called inkjet glossy papers, which are glossy media having high background gloss, or by subjecting an image formed part to gloss treatment using a transparent ink specialized for post-treatment as described in Patent Literature 1. However, when media having low background gloss, such as inkjet matte coated paper are inkjet-printed, image gloss is low, and high grade image is hard to be exhibited. When an aqueous ink is directly printed on commercial printing media such as A2 grade matte coated paper or A2 grade dull coated paper by the conventional aqueous inkjet system, the ink is not suitably absorbed in the media, causing image bleeding in general. When printing is performed with decreased amount of ink adhesion to prevent bleeding, image density is decreased and image quality cannot be exhibited. When the transparent ink specialized for gloss treatment is printed on the media, the ink is not suitably absorbed in the media, and a uniform and beautiful gloss comparable to that by offset printing cannot be obtained. Moreover, in the commercial printing field, printing cost is highly emphasized, therefore, it is not at all practical to use expensive inkjet media for catalogs, leaflets distribution and the like.
In a field of variable print, in which a part of an image is printed by offset printing beforehand, and then another image is merge-printed as necessary, the aqueous inkjet recording technology is expected to be applied. However, the inkjet recording cannot be suitably performed on the media, when both offset printing suitability and cost reduction are demanded to be achieved. Realistically, the aqueous inkjet recording technology can be applied in such a degree as to merge characters such as address. On the other hand, when the inkjet suitability of the media is enhanced so as to merge a beautiful image by the inkjet system, the offset printability of the media may be decreased, or the cost of the media may be increased. The inkjet suitability and offset printability are incompatible. Particularly, in the case of a production of a publication using a matte or dull medium, when an image is merged by the inkjet printing method afterward, the image has no gloss, which greatly differs from an image obtained by offset printing. Therefore, the aqueous inkjet recording technology in the field of variable print is used in very limited application areas and is hard to be applied in general commercial use in the current situation.
Patent Literature 2 discloses a method for obtaining image gloss by subjecting an image printed on a commercial printing medium to calender treatment. By this method, background gloss is also changed, thus, it does not satisfy the aim of exhibiting image gloss on a matte texture or dull texture.
Patent Literature 3 discloses an inkjet recording method using a pigment ink and a medium having a gloss at an angle of 60 degrees of 10% to 30% and a center surface roughness Ra of 0.6 μm to 4 μm. This method is aimed to prevent decrease of image gloss relative to a background, and intended for a matte gloss medium having relatively high background gloss. Therefore, this method does not meet an object of the present invention.
Particularly, in the case of a pigment inkjet ink, usually the image gloss is lower than the background gloss of a medium, which outstandingly occurs in glossy media, except that the image gloss is enhanced by bronzing of a coloring pigment. Many of conventional arts are aimed to improve the decrease of gloss. When the inkjet pigment ink is used to print on commercial printing media, offset due to color rub-off outstandingly occurs. This is because, in the case of the inkjet ink, a coloring pigment tends to remain on a medium surface, and a large amount of a resin cannot be contained in the ink, in comparison with an offset ink. Thus, the fixability of the pigment is poor, and serious problems occur with regard to images to rubbing, specifically, even though an initial image can be finely printed, the coloring pigment is separated during handling, or set-off occurs by stacking paper.
The inventors of the present invention have studied an inkjet recording method which enables to obtain a texture close to that of commercial printing at low cost, for the purpose of applying the inkjet recording method using an aqueous pigment in the commercial printing field.
In order to solve the above-mentioned problems, Patent Literature 4 suggests a method for forming images on commercial printing media at low cost by combining a pigment ink having high penetration ability and a recording medium that has low ink absorption ability by contrast with the conventional media. By this method, only a solvent (water or an organic solvent) for forming the ink is selectively infiltrated into the substrate by printing using a small amount of a pigment ink having ultrahigh penetration ability on a recording medium having provided thereon a coat layer for inhibiting ink absorption (penetration) so that the coloring pigment contained in the ink is not infiltrated intensively into the medium, and the coloring material (pigment) contained in the ink can be remained on the medium surface with good efficiency, without using any special material such as a cation fixing agent. As a result, both a sufficient density and drying ability can be realized with a small amount of ink. Moreover, because the coloring material contained in the ink is caused to remain effectively on the medium surface, high transparency of the layer that has been a necessary function in the conventional recording media becomes unnecessary. Therefore, the degree of flexibility of material configuration of the coat layer can be greatly increased. By applying this method, it is possible to perform inkjet recording even on paper with low ink absorption ability such as commercial printing paper or publication printing paper. This method enables to reproduce offset printing images on media having relatively high background gloss. However, on media having matte texture or dull texture, images can be printed but image gloss is hard to be exhibited. It has been difficult to reproduce so-called offset print image quality.
Patent Literature 1: Japanese Patent (JP-B) No. 3799995
Patent Literature 2: Japanese Patent Application Laid-Open (JP-A) No. 2004-62202
Patent Literature 3: Japanese Patent (JP-B) No. 3669314
Patent Literature 4: Japanese Patent Application Laid-Open (JP-A) No. 2007-144975